1. Field of the Invention
The present invention relates to an apparatus used for a surgical operation, in particular, to an apparatus used in a total knee replacement (TKR) operation and a total knee arthroplasty (TKA) operation for measuring a gap (joint gap) between osteotomized surfaces at a femoral distal end and a tibial proximal end as well as an inclination between the osteotomized surfaces (ligament balance).
2. Description of Related Art
In the knee replacement and arthroplasty operation, a replacement to knee prosthesis (tibiofemoral (T-F) joint) is done. Known in a prior art is a knee prosthesis, which includes a femoral component made of a metal material and fixed to an osteotomized surface at a femoral distal end, a tibial plate made of a metal material and fixed to an osteotomized surface at a tibial proximal end, and a surface component made of a plastic material such as a polyethylene and arranged between the femoral component and the tibial plate. In such a type of knee prosthesis, it is desired that a flexion and extension of tibiofemoral joint along a wide range of knee flex angle such as 150 degree must be possible. In view of this, the femoral component is smoothly curved such that the femoral component covers the entire osteotomized surface, which extends from an anterior part and a posterior part, of the femoral bone. In addition, the surface component made of the polyethylene material on the osteotomized surface at a tibial proximal end is formed such that the surface component has a shape, which is matched to the outer shape of the femoral component along the entire range of the knee motion.
During the execution of the total knee replacement operation, a precisely mounted condition of all of the components constructing knee joint is essential. In order to obtain such a condition, a possibility of a desired adjustment is essential as to a gap between osteotomized surfaces at a femoral distal end and a tibial proximal end as well as an inclination between these osteotomized surfaces. In view of this, during the execution of the surgical operation, a measurement of the gap as well as the inclination is done, while applying a tension of a predetermined value in a media and lateral collateral ligaments and in a patellar tendon, and, then, if a desired value of the gap or a desired value of the inclination is not obtained, a suitable relaxation or detachment of the media or lateral collateral ligament (soft tissues) attached to the femoral and tibial bones is done, thereby adjusting the gap or inclination or the both. Such a gap and inclination measurement as well as an adjustment of the same are, conventionally, done at both of extended and flexed conditions of the knee.
U.S. Pat. No. 5,911,723 of Ashby et al. proposes a measuring apparatus (so-called tensor) for measuring a gap between femoral and tibial osteotomized surfaces and a ligament balance, which apparatus includes a base having an engaging surface contacting with an osteotomized surface at a tibial bone and a moving head vertically movable with respect to the base and having an engaging surface contacting with an osteotomized surface at a femoral bone. The base and the moving head are connected with each other by means of a latchet mechanism. A measurement of the gap is done, while applying a predetermined value of load between the tibial and femoral bones, so that a measured value of the gap is displayed. In Ashby et al., the engaging surface of the moving head for a contact with the osteotomized surface at a femoral bone is rotatable about an axis, which is parallel with respect to the osteotomized surface. As a result, a ligament imbalance causes the engaging surface of the moving head to be inclined and the corresponding value of inclination (ligament balance) is able to be directly read out from a scale.
In Ashby et al., both of the tibial and femoral engaging surfaces extend, in a straight manner, anteriorly from the base and the moving head, respectively. Such a straight arrangement prevents the apparatus from being inserted between the femoral and tibial osteotomized surfaces without patellar eversion. Namely, in order to allow the apparatus to be installed, a patella eversion is done laterally from a medial skin incision line, thereby generating a space for an installation of the device. Furthermore, the patella eversion is maintained during the entire period for the measurement of the gap as well as the ligament balance.
As fully described above, in Ashby et al., it is essential that the gap and the ligament balance are measured while the patella eversion is maintained. Namely, with such a patella eversion, the patella is displaced or retracted laterally of the knee, thereby generating a space at the anterior side of the knee for the movable head and the base, which extend anteriorly from the upper and the lower engaging surfaces, which are engaged the femoral and tibial osteotomized surfaces, respectively. Such a measurement of the gap and ligament balance with the patella eversion is not desirable from the view point that a measurement during operation is done under a condition of the knee, which is quite different from a physiological condition of the knee after the completion of the operation.
Furthermore, in Ashby et al., the upper and the lower engaging surfaces are directly connected to the femoral and tibial osteotomized surfaces, respectively. A knee extension gap is measured as a gap between the osteotomized surfaces when a parallel relationship in anteroposterior direction is obtained between the upper and the lower engaging surfaces of the measuring apparatus. As far as the femoral distal end is concerned, the osteotomized surface is orthogonal (90° cut) to the longitudinal axis of the bone. The osteotomized surface at the tibial proximal end is, however, inclined (less that 10°) with respect to the axis of the bone in accordance with the shape of the tibial component. This means that the measurement in the Ashby et al. is influenced by the condition of the opposed osteotomized surfaces between the femoral bone and the tibial bone. Furthermore, the measurements at the extended condition (less than 180°) and the flexed condition (90°) are only possible, i.e., Ashby et al. does not make it possible to measure at any desired knee flexion angle.